Positioning mechanism for specimen inspection and processing

ABSTRACT

A specimen positioning mechanism includes a movable stage movable along multiple axes, a plate connected to and supporting a specimen mounting chuck, multiple linear displacement mechanisms coupling the plate to the movable stage and mutually spaced apart at different locations between the movable stage and the plate and separately controllable to change distances between the movable stage and the plate, and a flexible member coupling the movable stage and the plate. The flexible member is motion compliant in three axes of motion. The flexible member in response to linear displacements of the linear displacement mechanisms allows linear and rotational movement of the specimen mounting chuck in the three axes of motion compliance.

RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 10/891,719,filed Jul. 14, 2004, now U.S. Pat. No. 6,891,601, which claims benefitof U.S. Provisional Application No. 60/488,141, filed Jul. 17, 2003.

TECHNICAL FIELD

This invention relates to the field of semiconductor processing devicesand, more particularly, to a system for dynamically aligning a wafer inZ-, tip-, tilt-, and yaw- (theta) axes relative to a wafer processingdevice.

BACKGROUND INFORMATION

There are various prior Z Tip Tilt (“ZTT”) devices for adjusting theheight and parallelism of a semiconductor wafer in a semiconductorprocessing machine. ZTT devices typically control positioning of Z-axisdisplacement, rotation about an X-axis, and rotation about a Y-axiswhile the semiconductor wafer is moving in the X-Y directions under asemiconductor processing machine, such as an optical inspection system.The ZTT device dynamically compensates for non-flatness of the wafer andshould be stiff to provide high bandwidth positioning.

Typical ZTT devices are mounted on an X-Y positioning stage and shouldbe sufficiently lightweight and compact to maintain the dynamicperformance of the X-Y stage. The ZTT positioning device should also beaccurate within a few nanometers, be geometrically stable, and have asensitive and repeatable driving system. Moreover, ZTT devices shouldprevent contact between the wafer and the processing system, should notgenerate particles that could contaminate the wafer, and should besufficiently reliable to maintain wafer processing throughput.

A conventional approach for providing ZTT positioning integrates two ormore separate technologies or products, such as mechanically splittingthe Z-axis (vertical) positioning and the tip and tilt positioning, anapproach which typically results in very large, high profile, high-massmechanisms. When splitting the Z-axis and tip/tilt positioning, the mostcommon approach maintains a fixed wafer Z-axis position and, instead,moves the wafer inspection/processing elements. This approachcomplicates the design of the inspection/processing elements (typicallya multi-element optical assembly) and increases the risk of particulatecontamination because the vertical translation stage is typicallylocated directly above the wafer. Also, because the moving mass of theZ-axis translation stage (and the elements it carries) is greater thanthat of a wafer chuck, the resulting dynamic performance is inadequatefor many high-throughput applications.

Another conventional approach also mounts the tip and tilt positionersabove the wafer. A problem with this approach is maintaining co-locationof the inspection/processing system focal point and the tip and tiltpositioner axes to prevent X-Y translation of the inspection/processingpoint as the tip and tilt angles are changed. Of course, mass,complexity, and contamination risk remain problems with this over-waferconfiguration.

Several conventional approaches exist for providing tip and tiltpositioning beneath the wafer chuck, such as on the X-Y stage carriage.For example, stacking two goniometric cradle stages with coincidentrotational axes provides tip and tilt rotation about a common pointlocated at the wafer surface. This approach provides relatively largetip and tilt positioning angles but is problematic because it employsmechanical bearings and drive screws, has a high profile, and cannotdirectly measure the tip and tilt angles. Alternatively, this cradleapproach may be further coupled to a Z-axis stage that is also locatedon the X-Y stage carriage. The most common conventional Z-axis stagesfor mounting to an X-Y stage employ either a horizontal wedge driven bya mechanical actuator or linear motor, a single drive screw with avertical guide way, or three or four small vertical drive screws thatturn synchronously to provide Z-axis movement. All these approaches areoverly tall and massive to achieve suitable dynamic performance in highthroughput applications.

Another conventional tip and tilt positioner approach employs flexuremechanisms driven by mechanical or piezo-electric actuators connected toa support plate that rests on a pivot point defining the center of tipand tilt rotation. In this approach, two identical flexures spaced apartby 90 degrees and at a same radius from the pivot point, providerotation about one axis and translation along another axis. Thecombination of rotation and translation creates the tip and tiltpositioning. However, the flexures must be compliant through therotational axis while providing stiffness for the mechanical structure.This tradeoff limits either rotational range or stiffness.

Another conventional flexure approach employs a single stage thatprovides tip, tilt, and a small amount (less than 1 mm) of Z movement,by simultaneous actuation of two opposing flexures. This approachemploys four flexures, a support plate, but no centered pivot point. Thefour flexures are spaced apart 90 degrees around the circumference ofthe support plate. Tip and tilt movement is provided by actuating twoopposing flexures in opposite directions. Z-axis movement is provided byactuating all four flexures in the same direction. This approach alsosuffers from limited range or a lack of mechanical stiffness.

In addition to ZTT positioning, many wafer processing applications alsorequire rotational angle (theta) positioning about the Z-axis. Thetapositioning typically includes static “fine theta” adjustments foraligning a wafer when it is loaded on a chuck and “dynamic theta”adjustments for maintaining alignment during movements of the X-Y axispositioner. The fine and dynamic theta positioners are typically mountedon the X-Y positioning stage. The fine theta positioner should be closeto the wafer to avoid X-Y errors, whereas the dynamic theta positionershould be mounted at a lower position to compensate for parasiticrotations of the wafer.

As with the ZTT positioners, the fine and dynamic theta positionersshould be lightweight, compact, and stiff to provide suitable dynamicperformances; accurate to within a few nanometers; stable, sensitive,and repeatable; should not generate wafer contaminating particles; andbe sufficiently reliable to maintain machine throughput.

A common conventional theta positioner employs a mechanical rotary stagemounted to the X-Y positioning carriage. Such a rotary stage includes arotating carriage supported by a worm-gear driven radial bearing set.Alternatively, a direct-drive torque motor may drive the stage. However,the mass, height, and inherent mechanical properties of the bearingstage compromise the X-Y stage performance. Moreover, achieving adesired zero-dither performance for the theta stage requires adding abrake or locking mechanism to the stage, which further increases themass and complexity of the positioner.

A solution for providing suitable theta positioning performance employsa simple two-plate air bearing structure in which a flat reference plateis mounted to the X-Y stage carriage. An upper plate having pressure andvacuum orifices is installed above the reference plate forming an airbearing gap between the two plates. The upper plate is tangentiallydriven by a linear actuator on one end and is supported by a rigidflexure mechanism on the opposite end to form a pivot point for thetheta adjustment. After adjustment, the air bearing pressure supply isblocked, allowing the remaining vacuum to adhere, and thereby lock, theupper and lower plates together. However, because the stage is locked,it cannot provide the dynamic theta adjustments required by someapplications. Moreover, the travel range of this approach is limited bythe rigid flexure mechanism and by a lateral shift that occurs betweenthe actuator contact point. Another disadvantage of this approach isthat the center of rotation is offset from the X-Y carriage center,making it necessary to compensate in X-Y for the theta offset angle.

A solution for providing both very fine theta adjustment within aboutone degree and high-bandwidth response employs differential positioningof two parallel stages connected by a single perpendicular stage. Thisapproach, referred to as an H-bridge configuration, employs flexures ateach end of the single perpendicular stage to allow a small amount ofindividual mechanical movement between two connected parallel stages.This movement creates an offset angle of the single stage with respectto the parallel axes and, in turn, the desired theta offsetfunctionality. While this solution adds little hardware to the X-Ysystem to provide theta functionality, it still has a limited travelrange and provides no way to lock the theta position. High-bandwidththeta adjustments are possible with the H-bridge configuration, butbecause flexures are needed to accommodate the differential movement ofthe parallel stages, the dynamic response of the X-Y stage is reduced bythe flexure compliance.

SUMMARY OF THE INVENTION

An object of the invention is, therefore, to provide a wafer positioningstage that provides Z-axis, tip, and tilt positioning in a singlemechanism that is integrated with the X-Y carriage without compromisingthe dynamic performance of the X-Y stage or related system elements.

An advantage of the invention is that it also provides fine and dynamictheta positioning with fine adjustment capability, moderate travelrange, high-bandwidth response, zero angular dither at any desiredposition, negligible influence on X-Y stage throughput, and angularrotation through the X-Y carriage rotational center.

A ZTT positioner of this invention employs a flexible disk that allowsZ-axis displacement and tolerates tip and tilt rotations. The disk hasminimum mass, stiffness in the X and Y directions, and high damping toavoid vibration. A driving system employs three non-contacting voicecoil motors each having a spring to compensate for the moving mass.Position feedback is provided by non-contacting linear encoders coupledto each voice coil motor. The motors and encoders are mutually angularlyspaced apart 120 degrees around the circumference of the disk to providehigh sensitivity and accuracy.

The ZTT flexible disk includes multiple laminated plates. The upperplate is formed from a very stiff, low mass, ceramic material. Theinterface to the X-Y stage depends on the application, but could includea theta stage for angular alignment, a lift pin mechanism, and a waferchuck. The ZTT positioner further includes adjustable hard limits toprevent contact between the wafer and the processing system.

Fine and dynamic theta positioners of this invention together providefine adjustment capability, moderate travel range, high-bandwidthmechanical response, zero angular dither at the desired position,negligible influence on the X-Y stage throughput, and angular rotationthrough the center of the X-Y stage. The theta positioner is preferablyintegrated with the ZTT positioner.

The fine theta positioner employs an air bearing rotary stage with acentered pivot point to allow rotation through a few degrees. The airbearing rides on air pressure that is preloaded with a vacuum. Afterfine theta alignment, the pressure is shut off, thereby vacuum clampingthe fine theta mechanism to a reference surface. The clamping provides avery stiff mechanism having minimum size and mass. The air bearingemploys three air pads with an integrated interface for mounting thewafer chuck. The fine theta driving system employs a non-contactingvoice coil motor. The angular feedback is provided by a non-contacting,high resolution angular encoder. During clamping, the motor and encoderare in a closed-loop configuration to ensure accurate angularpositioning.

The dynamic theta positioner employs a flexible pivot driven by threepiezo actuators spaced apart 120 degrees about a pivot point. Theflexible parts are oriented to focus rotation about the pivot point,thereby avoiding parasitic X-Y displacements during angular rotation.The three flexible mechanisms each have a small size, but are mounted ata large radial distance from the pivot point to provide high ZXYstiffness even when loaded with several kilograms. The dynamic thetapositioner is substantially frictionless and is clean and reliable.

Additional aspects and advantages of this invention will be apparentfrom the following detailed description of preferred embodiments, whichproceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial side elevation view of a preferred embodiment of aZTT-theta positioner of this invention.

FIG. 2 is an exploded isometric pictorial view of the ZTT-thetapositioner of FIG. 1 showing major ZTT positioner components.

FIG. 3 is an isometric pictorial view of a flexible disk component ofthe ZTT-theta positioner shown in FIG. 2.

FIG. 4 is an exploded isometric pictorial view of flexible disk, ceramicplate, voice coil motor, and linear encoder scale components of theZTT-positioner.

FIGS. 5A and 5B are respective enlarged isometric and side sectionalviews depicting in greater detail adjustable hard Z limit components ofthe ZTT voice coil motor components shown in FIG. 4.

FIG. 6 is an exploded isometric pictorial view of fine theta positioningcomponents shown in FIG. 1.

FIG. 7 is an exploded isometric pictorial view of dynamic thetapositioning components of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 show respective side elevation and exploded views of apreferred embodiment of a ZTT-Theta positioner 10, which is assembledbetween the top of an X-Y stage 12 and the bottom of a wafer mountingchuck 14. X-Y stage 12 moves in X- and Y-axis directions relative to aflat surface 16, such as a granite slab. ZTT-Theta positioner 10 ismounted to an upper surface 18 of X-Y stage 12 and acts to accuratelymove chuck 14 in the Z-axis direction, tip (roll) chuck 14 about theX-axis, tilt (pitch) chuck 14 about the Y-axis, and rotate (yaw) chuck14 about the Z-axis. Accordingly, chuck 14 undergoes six-axes ofcontrolled movement in the X, Y, Z, roll, pitch, and yaw directions.

ZTT-Theta positioner 10 is a low-profile assembly occupying only about35 mm of the total 115 mm height of X-Y stage 12, positioner 10, andchuck 14. X-Y stage 12 is electrically connected to a controller (notshown) by a flexible cable 20. The low-profile reduces angular torque bykeeping the mass as low as possible and limiting the amount ofrotational inertia.

FIG. 3 shows a flexible disk 22 that acts as a guideway for ZTT-Thetapositioner 10 by allowing Z-axis displacements and tip and tiltrotations while having high stiffness in the X-axis, Y-axis, and thetadirections. Flexible disk 22 is preferably divided into three fixedsectors 24 that includes openings 25 and static mounting points 26 andthree movable sectors 28 that have movable mounting points 30. Mountingpoints 26 and 30 are preferably all at the same radial distance from therotational center of flexible disk 22. Openings 25 are preferablytriangular with rounded corners to reduce stiffness of flexible disk 22and equalize flexure of sectors 24 and 28. Equalizing the flexureprovides a same amount of displacement relative to the radius at thepoints of attachment. Static mounting points 26 are also formed inmounting interface members 31, which are in the form of annular segments(FIG. 4) and are fitted on the top and bottom sides of static mountingregions 26 of fixed sectors 24 to secure them either directly orindirectly to upper surface 18 of X-Y stage 12. Slits 32 extendingradially from the center to points near the periphery of flexible disk22 intersect the shorter side boundaries of mounting interface members31. Slits 32 define boundary lines between and permit relative movementin three axes (Z, roll, and pitch directions) of adjacent fixed sectors24 and movable sectors 28. Mounting interface members 33 in the form ofannular segments (FIG. 4) radially extending from a common hub includeapertures 34 that are axially aligned with movable mounting points 30.Fasteners (not shown) extending through apertures 24 and movablemounting points 30 secure movable sectors 26 to an upper plate 35 (FIGS.2 and 4) that supports chuck 14.

Upper plate 35 is preferably formed from silicon carbide (SiC) ceramicmaterial to provide low mass, high stiffness, and low thermal expansion.Sectors 24 and 28 of flexible disk 22 are optimized in size and positionto provide a high stiffness in the X, Y, and theta directions. Flexibledisk 22 is preferably a multilayered structure that is composed ofseveral thin steel disk elements bonded together with double-sided tapeto provide a high damping factor to avoid vibration and improve the ZTTmovement bandwidth. A motive force necessary to provide suitable Z-axisdisplacements is substantially lower than the force required with asingle thick disk. Although flexible disk 22 has a relatively lowdisplacement range, it is very reliable because there is no stress inthe steel and the double-sided tape bonds large surfaces. Moreover,flexible disk 22 is very clean and operates without lubrication.

FIG. 4 shows a ZTT driving system that movably couples upper plate 35 toX-Y stage 12. The driving system employs multiple extensible mechanisms,preferably three voice coil motors including motor magnets 36 that arefixedly mounted on X-Y stage 12 and motor coils 38 that are attached toupper plate 35. There is no contact between motor magnets 36 and motorcoils 38, resulting in a direct drive arrangement between the movingupper plate 35 and X-Y stage 12. As indicated in FIG. 4, motor magnets36 and motor coils 38 pass through triangular shaped cutout regions 40in flexible disk 22 and make no contact with it. Reliability is improvedby making the gap between motor magnets 36 and motor coils 38sufficiently large to avoid contact when the tip and tilt angles are ata maximum. The moving mass is compensated for by coil springs 42 thatare located around and pass through cutout regions 42 in flexible disk22 to motor coils 38 provide force against X-Y stage 12. Becauseflexible disk 22 does not generate significant force over its five mmtravel range, coil springs 42 efficiently reduce the electrical currentrequired by motor coils 38, thereby reducing temperature rise andincreasing the thermal stability of the ZTT mounting interfaces.

ZTT-Theta positioner 10 provides a ±2 mm Z-axis travel range with 70 nmrepeatability and a 5 μm step and settle time of 40 msec. ZTT-Thetapositioner 10 also provides ±0.5 mdegree tip and tilt rotational rangeswith 2 μradian repeatability. Alternatively, extensible mechanismsincluding short stroke linear motors (of which voice coil motors are ofone type) and piezoelectric mechanisms may be employed.

ZTT position sensing is provided by three linear optical encoders, eachof which includes an optical sensor head 44 and a linear scale 46 (FIG.5A), optical sensor heads 44 are mounted adjacent to motor magnets 36 onX-Y stage 12, and linear scales 46 are mounted adjacent to coil springs42 on upper plate 35. Linear scales 46 have a large range of angulartolerance that allows accurate Z displacement measurements when the tipand tilt angles are maximum. The three Z positions are sufficiently wellknown to provide a Z-axis translation resolution of 20 nm.

The three motor coils 38 and linear scales 46 are mounted inheight-reducing recesses formed in upper plate 35. Metallic inserts inthe recesses provide high stiffness mounting surfaces. The mountingsurfaces are also very accurate and flat because of the ceramic materialprocesses forming upper plate 35. Upper plate 35 further includes thenecessary interface for mounting an air bearing rotary stage and a liftpin mechanism for wafer leveling on chuck 14 and optional thetaalignment mechanisms that are described with reference to FIGS. 6-8.Upper plate 35 also includes optional interfaces for mounting anyancillary components necessary for particular wafer processes. Finally,reference mirrors may be mounted on upper plate 35 to accommodateinterferometer-based X-Y positioning measurements.

FIGS. 5A and 5B show respective isometric and side sectional views of aZ-axis movement adjustable hard limit mechanism 50 of this invention.ZTT-Theta positioner 10 preferably employs three hard limit mechanisms50, each of which is integrated with the supports for motor magnets 36and motor coils 38. A static fork 52 is mounted on a motor coil support54, and a moving fork 56 is mounted on a motor magnet support 58, whichis attached to upper surface 18 of X-Y stage 12. The guideway of movingfork 56 is a pneumatic jack 60 that includes a clamping capability. Aspring 62 preloads moving fork 56 to provide Z-axis downwarddisplacement bias. Motor magnets 36 and motor coils 38 provide thedriving motive force for adjusting the position of moving fork 56.

During the adjustment process, moving fork 56 is unclamped and drivenupward by pneumatic jack 60 against the urging of spring 62. Meanwhile,the ZTT controller moves motor coils 38 and thereby static forks 52 tothe commanded upper hard limit, at which position moving forks 56 areclamped by pneumatic jack 60. The upper hard limit positioning is veryprecise because it employs ZTT motor coils 38 and its associated linearscale 46. Accordingly, Z-axis displacement can be very close to theupper hard limit. Hard limit mechanisms 50 are equally spaced about theperiphery of ZTT-Theta positioner 10, the diameter of which issufficiently close to the wafer diameter to avoid Z-axis offsets in thepresence of tip and tilt angles.

ZTT-Theta positioner 10 optionally includes fine and dynamic thetapositioner mechanisms.

FIG. 6 shows a fine theta positioner 70 of this invention that includesa flexible disk 72 formed as a multilayered structure that is composedof several thin steel disks are bonded together with double-sided tapeto provide a high damping factor. Flexible disk 72 provides stiffness inthe X, Y, and theta directions and efficient decoupling in the Z-axisdirection.

Flexible disk 72 includes three arms 73 mutually angularly spaced apartby 120 degrees. The end of each arm includes an air pad 74, which expelsfrom its periphery pressurized air to form an air bearing region forfrictionless movement of air pad 74 across a reference surface 76embedded in upper plate 35. Within reference surface 76 is a vacuum port77 that provides an offsetting vacuum pressure bias that is slightlyless than the air pressure creating the air bearing. When the airpressure is interrupted, the vacuum pressure dominates and clamps airpad 74 to reference surface 76, thereby locking in the currentlyselected fine theta positioning angle. The air bearings also contributeto improved reliability, and the high damping factor of flexible disk 72avoids vibrations, reduces parasitic forces on air pads 74, and improvesfine theta positioning bandwidth.

A ball bearing 78 fitted into a pedestal mounted on upper plate 35 ridesin a centered pivot point 80 fitted in a hub 81 of flexible disk 72 thatdefines the center of theta rotation. The three air pads 74 are affixedand thereby linked to ball bearing 78 by flexible disk 72.Alternatively, air pads 74 may include ports for both the pressurizedair and vacuum pressure, or some combination of permanent-magnets,electro-magnets, and springs may provide suitable attracting and/orrepulsing forces.

The fine theta driving system employs a voice coil motor that includes amotor coil 82 that is attached outboard of one of air pads 74 and amotor magnet 84 that is attached to X-Y stage 12. The maximum radialposition of motor coil 82 provides sufficient torque for the small, lowmass voice coil motor. The voice coil motor provides non-contacting,direct drive between air pads 74 and X-Y stage 12. Reliability isincreased by making the gap between motor coil 82 and motor magnet 84sufficiently large to avoid contact when the fine theta angle ismaximized.

Fine theta position feedback is provided by a rotary encoder thatincludes an optical sensor 86 that is mounted on X-Y stage 12 and anencoder scale 88 that is mounted outboard of one of air pads 74. Therotary encoder provides direct angular information of the fine thetaangle. Encoder scale 88 employs a Renishaw encoder supporting less thanfive pradians of resolution across ±3 degrees of rotation.

Fine theta positioner 70 includes an angular clamping capability. Duringangular alignment, the air bearing is pressurized and there is,therefore, no friction to impede a sensitive, accurate angulardisplacement. When the target angular position is reached, the airpressure is cut off, allowing the vacuum to clamp air pads 74 toreference surfaces 76. The high preload of the vacuum ensures a stiffand stable theta angle relative to X-Y stage 12. During clamping, thecontroller servo loop is closed to ensure an accurate target alignmentangle. After clamping, the servo loop is opened to eliminate currentflow through motor coil 82, thereby eliminating heat generation toensure thermal stability.

Each of air pads 74 further includes a chuck mounting interface 90composed of a cone and a ball that decouple theta stresses from chuck14. Chuck 14 is rigidly affixed to air pads 74 by screws.

FIG. 7 shows a dynamic theta positioner 100 of this invention thatincludes three static bases 102 mounted at 120-degree intervals on areference surface, such as X-Y stage 12. Dynamic theta positioner 100further includes three movable bases 104 that are interconnected tostatic bases 102 by flexures 106. Flexures 106 provide decouplingbetween static and movable bases 102 and 104, which are linked togetherby piezo actuators 108. Movable bases 104 are coupled to static points26 of flexible disk 22 (FIG. 3). When optionally installed, dynamictheta positioner 100 renders unnecessary and therefore replaces theflexible disk mounting interface 31 shown in FIG. 4.

The orientation of the decoupling between static and movable bases 102and 104 is directed radially toward a centered pivot point 110 thatprovides accurate theta rotation without X-Y parasitic displacement.Flexures 106 are optimized to ensure high stiffness in the X-Ydirections. The spacings between adjacent ones of the three pairs ofinterconnected static bases 102 and movable bases 104 are sufficientlylarge to provide high stiffness in the Z-axis, tip, and tilt directions.The spacings also provide a free area 112 for integrating otherfunctions, such as ZTT, fine theta, and an optional lift pin mechanism114 for assisting wafer loading on chuck 14. Lift pin mechanism 114integrates with fine theta positioner 70 (FIG. 6) and includes threetubular vacuum supply lines 116 that extend upwardly through holes 118in chuck 14 (FIG. 2). Lift pin mechanism 114 provides vacuum supplylines 116 with about 6 mm of travel for vacuum gripping and movingwafers to and from chuck 14. All other integrated functions are mountedabove dynamic theta positioner 100 to ensure proper dynamic thetaalignment.

Piezo actuators 108 are preloaded for displacement in forward andreverse direction without hysteresis. Piezo actuators 108 includeintegrated position sensors to provide accurate displacements withoutdrift and hysteresis.

Angular position feedback may be provided by the optical system thatmeasures wafer alignment during XY displacement, or be provided by aninterferometer having reference mirrors mounted close to chuck 14 or thewafer. In either alternative, dynamic theta positioner 100 providesdynamic rotation of the wafer to within 0.5 μradian across an angulartravel of ±10 μradians.

Referring again to FIG. 2, shown are the spatial relationships amongvarious components of the ZTT positioner, fine theta positioner 70, anddynamic theta positioner 100 of this invention.

Skilled workers will recognize that portions of this invention may beimplemented differently from the implementations described above forpreferred embodiments.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments withoutdeparting from the underlying principles of the invention. The scope ofthis invention should, therefore, be determined only by the followingclaims.

1. A high resolution, dynamic specimen positioning mechanism,comprising: a movable stage operable for movement in directions alongmultiple axes; a plate operatively connected to a specimen mountingchuck to support it; multiple linear displacement mechanisms couplingthe plate to the movable stage, the linear displacement mechanismspositioned in a space between the movable stage and the plate andcontrollable to change distances in the space between the movable stageand the plate; a flexible member operatively connected to the movablestage and the plate, the flexible member being motion compliant in threeaxes of motion, one of which defines a direction of linear displacementof the linear displacement mechanisms, and the flexible member inresponse to linear displacements of the linear displacement mechanismsallowing linear and rotational movement of the specimen mounting chuckin the three axes of motion compliance; a dynamic positioning mechanismincluding first and second base portions operatively coupled to,respectively, the movable stage and the flexible member; and a drivemechanism operatively coupled to the first and second base portions toimpart relative movement to them and thereby angular displacement of thespecimen mounting chuck about the direction of linear displacement.